Use of "N" for coasting

The real story is that it is possible to damage MG1 by coasting in N. You really have to work at it with a very unlikely set of circumstances, but it can be done. If you shift to N with the ICE off, the ICE cannot start. If you then coast down a *very* steep hill it is possible to force MG1 to spin too fast, since the ICE cannot spin to relieve the overspeed. It's not likely, especially since you now know the secret.

Shifting to P or R while moving causes the car to shift into N, which returns us to the above discussion.

Tom

 

One difference is that coasting in neutral is illegal in most US states. Not sure how it could be enforced unless you had a wreck and the Prius' little black box ratted you out.

 

For my TCH, if put in N to coast, the ICE will spin when speed is above 40 MPH.
So the ECU will protect MG1 if MG1 spins too high by keeping ICE spinning and fuel is consumed.

 

At 55 mph if you shift to N, then the engine will be started by the ECU to protect MG1, you will then consume gas whereas leaving it in D and feathering the pedal would only result in a very small loss in glide due to the engine running without gas (but with very limited friction compared to conventional ICE cars).
This is the difference in your scenario.

The danger of N gliding is in this scenario:
You crest a hill under 45 mph and shift to N. Because we're still under MG1 max speed, the engine is not running at all. Then if you glide downhill and accelerate past 50mph, this is where you risk damaging MG1, at least that is what I understood after reading numerous posts here on pc.com describing this issue.

Therefore my simple rule of thumb is, I never shift or let the sticker in N if the speed gets higher than 45 mph.
In other words, N-gliding is only for sub-45mph speeds.

 

double-post due to pc.com's server...

 

It is a simple calculation of distance divided by fuel consumed. A warmed up engine will burn very little fuel and at highway speeds, considerable distance is being covered.

If your engine is turning over, some power must overcome the piston and mechanical resistance. There is no 'free lunch' even if battery power is used to keep the ICE turning over.

The engine drives the carrier of the planetary gears which are sandwiched between the star, MG1, and ring, MG2 and wheel drive gears.

If the engine is stopped, motion from the ring gear will pass via the planetary gears to the sun gear and spin MG1 very, very rapidly. In theory, it can spin fast enough that MG1's rotor will break apart from centrifical forces. However, other analysis suggests a possible earlier failure mode.

As MG1 rotor spins faster and faster, it generates a higher and higher voltage. At a high enough voltage, it could over stress the power electronics and short out the inverter.

So far, I have yet to see the 'forbidden' experiment performed to determine which happens first, inverter electrical failure or rotor torn apart. But keep us informed about your experiments. <grins>

Personally, I follow these rules:

• below hybrid threshold (42 mph NHW11, 46 mph ZVW30) - "N" is no problem.
• above hybrid threshold - "N" is no problem.
• from below hybrid threshold to above - hit "D" briefly above threshold to get ICE running and no problem
• from above hybrid threshold to under - hit "D" briefly so ICE can stop

Otherwise, drive the conditions and be sensible to other traffic.

Bob Wilson

 

Correcting a few errors or confusions from the above posts:

1) The MPG display is only interested in gasoline consumption. A freewheeling ICE does not burn gasoline, so it does not influence the displayed mileage. It does consume some energy due to friction, but that comes from the HV battery and is not calculated as part of the mileage display. While not technically correct, the Prius ignores the energy stored in the HV battery for all mileage calculations. Since the capacity of the battery is relatively low, this is a fairly reasonable practice.

2) The ICE does not need to be "switched on" to keep MG1 from over-speeding. Field voltage is applied to MG1 to retard its speed, which forces the ICE to freewheel due to the mechanical coupling of the planetary gearing.

3) For the OP: We answered your question in the first couple of posts. Coasting in N will not damage your Prius as long as you avoid the rather unlikely set of circumstances where you shift into N a low speed and then coast to very high speeds.

Tom

 

While the vehicle is in "N"?

I've monitored MG1, MG2 and ICE rpms using a Graham miniscanner with our NHW11 and haven't seen that effect while coasting up to 50 mph in "N". I'm waiting on the ZVW30 release of Auto Enginuity to gather metrics for that model.

I don't have an NHW20 for testing. Does this field effect work in the NHW20s in "N"?

Bob Wilson

 

Perhaps my wording wasn't clear. The point I was trying to make is that the Prius doesn't have to power up the ICE to keep MG1 from spinning too fast. All it has to do is slow down MG1, which forces the ICE to freewheel. As we all know, MG1, the ICE, and MG2/wheels are all permanently connected through the PSD, which is just a small fixed planetary gearbox. For any one of the three parts of the PSD to turn, at least one other part must also spin. When you coast in N the wheels/MG2 are spinning. If the ICE is stationary, that means that MG1 must spin. If MG1 is stationary, then the ICE must spin. Likewise they both can spin, but at a reduced rate.

MG1 freewheels very easily. It's just a small electric motor with very little frictional drag. The ICE, on the other hand, has a lot of moving parts and compression to overcome. It doesn't easily freewheel. If you spin MG2/wheels without doing anything else, the ICE will sit still and MG1 will spin.

To keep MG1 from over-speeding in a high speed coasting situation, you have to spin the ICE. This can be done in two ways: 1) Apply gas and spark to the ICE so that it spins itself, or 2) Apply drag to MG1 so that the ICE will freewheel.

The way you add drag to MG1 is to apply a field current in opposition to the coasting rotation. If you lock MG1 with the field, all of the rotation will go to the ICE. If you merely retard MG1, both the ICE and MG1 will spin. Retarding MG1 wastes power in MG1. Allowing the ICE to spin wastes power as friction. Pick your poison.

As to when the Prius retards or locks MG1 verses powering the ICE, only testing or documentation would tell.

Tom

 

This is the $64 question (or perhaps in this case the $6400 question). I agree 100% that theoretically the MCU could prevent overspin on MG1 by applying a voltage drag and causing the ICE to freewheel, but does it? I think what Bob was suggesting is that he has experimented and has never seen this behaviour in his NHW11 and overspinning MG1 is a risk. Can anyone confirm what the ZVW30 will do?

 

The Prius does prevent over-speed of MG1 when in D. The Prius does *not* change the state of the ICE when in N, which is why MG1 over-speed is a risk for high speed coasting. If the ICE is already turning, it will continue to turn. If the ICE is stationary, it will not activate in N. This appears to be true for both the Gen II and Gen III.

Tom

 

Thanks, of couse it's a given when in D. I was referring to the OP's original postulation - coasting in N. Makes me wonder why Toyota would do this. Given the already sophisticated programming required to run the Prius, it seems that it would be trivial to have it at least prevent damaging overspin on MG1 when in neutral...

 

Thanks for the clarification. I thought the original poster was after "N" effects. I have no doubt the ICE can be spun with MG1 power, this is how "B" works. I quite agree.

Bob Wilson

 

That wastes gasoline.
When you are coasting downhill in D at 50 mph, you are in fuel-cut mode.
If you shifted to N, then the ICE continues to run using some fuel to maintain the idling speed.

On my observation, Gen2 seemed not to have prevent damaging overspin on MG1.

Ken@Japan

 

In fact, I would add that I've seen the ICE rpm increase when the speed increased descending a hill while in "N" (ICE already running when shifting into "N".) The ICE controller did what it needed to increase ICE rpm and keep MG1 in the proper speed range. Neat trick considering there is no MG1/MG2 torque available.

Bob Wilson